Isothermal fatigue behavior of the near-eutectic Sn-Ag-Cu alloy between -25°C and 125°C

When near-eutectic Sn-Ag-Cu (SAC) alloys are used to make soldered ball-grid-array (BGA) assemblies, the grain size of the joints is very large. During thermomechanical cycling, the solder joint fatigue process is often initiated with recrystallization of the Sn grains, resulting in a smaller grain size in the deformed areas. Grain boundary sliding and increased grain boundary diffusion then results in intergranular crack nucleation and propagation along the recrystallized Sn grain boundaries. In this work, fatigue tests were used to study the initial stages of cyclic deformation damage in Sn-Ag-Cu alloy samples. To separate the solder properties from the constraints introduced by the substrate, the tests were done with free-standing solder specimens, instead of solder joints. The test samples were cast dog-bone specimens that have a cross-sectional diameter of 1 mm, which corresponds to a typical solder joint diameter in BGAs. Mechanical cycling was performed isothermally at several temperatures, from -25°C to 125°C. Typical test conditions were ± 1.5% strain and 15-minute hold at tensile peak stress to allow stress relaxation to take place. Optical microscopy, scanning electron microscopy, and electron back-scattering diffraction were used to study the microstructures of the samples before and after fatigue testing in order to obtain insight into the nucleation and growth mechanisms of fatigue damage. [PUBLICATION ABSTRACT]